In recent years, attention has been paid to technology for detecting a difference between two or more images such as event detection from a moving image for monitoring, abnormality detection in medical use, and change detection from an aerial or satellite image. Techniques for detecting the presence or absence of an event from a moving image have been proposed (for example, see Patent Document 1). In such techniques, whether or not there is a change in a target image is detected on the basis of a ratio between a complexity index value of a target image (calculated from a quantization step width and a generated code amount) and a complexity index value of an immediately previous image. That is, it is possible to detect a change in images in a time axis direction and the above-described technique can be applied to event detection from a monitoring moving image.